Ultra chess!
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How would you prevent such a large game from becoming very boring? The Kings will be hiding behind a thick wall of pieces, so that you would have to trade away some 60 pieces before you even get a possibility to check. Some of these pieces being very slow, and needing 10 moves each to meet each other in order to be traded. So it might take a thousand moves before the first check can occur...
15 pawns
8 rooks
14 bishops
14 knights
5 queens
1 king
7 archbishops
6 camels
2 Amazons
And put them anywhere on a 20x20 board! (Both players can have different setups)
No castling and no pawn don’t move 2 on their first turn. (No en passant)